Thirty seconds into his first stint of playing time, Brown burst past Dario Saric and Robert Covington to retrieve a loose ball, beat everyone down the court and slam home a high-flying dunk. During the sequence, early in Boston’s 108-103 Game 2 win, Brown appeared unbothered by his strained hamstring. He looked, as usual, like one of the NBA’s most athletic players.

But the troubling part arrived after Brown landed from his powerful dunk. He winced, hopped on his leg, and barely celebrated his loud bucket. After the 76ers stopped play with a timeout, Brown gave two meek high fives, chatted briefly with teammate Marcus Morris, and retreated to the Celtics huddle, where a number of coaches and trainers were wondering if his hamstring had held up. A look at Brown’s face showed the issue was bothering him, but he opted to stay in the...